Optical fiber-coupled components transmit digital or analog data over optical communications networks at time-varying power levels. Notwithstanding such variation, it is usually desirable for the average transmitted power to remain substantially constant. In the absence of such average power-level stability, problems relating to exceeding maximum or minimum input levels on other network components may arise. Moreover, significant power variations between multiple data channels can result in unacceptable levels of cross-talk therebetween. As network complexity increases, such as with the addition of active switching, e.g., wavelength add-drop and cross connect, the potential for problems relating to signal power variations increases.
Typically, intervention is required to maintain substantially constant average transmitted power levels. Such intervention is necessitated because average power transmitted by a given component will vary as a result of component degradation, changes in the fiber-coupling efficiency, or a change in input to the component. Intervention may be implemented as a system that includes an electrically-controlled variable optical attenuator used in combination with an optical signal monitor.
In a conventional implementation of the aforementioned system, a variable attenuator receives an optical input, attenuates the optical input, and delivers a portion of the received optical input. The delivered amount of signal is a function of the variable attenuator's then current attenuation setting. The delivered portion is received by an optical tap. Most of the delivered portion is transmitted by the optical tap to an output for delivery to subsequent network components. The signal transmitted to the output is the controlled quantity, and it is that signal that is delivered, under the best conditions, at a constant average power level. The small amount, e.g., about one percent, of the delivered portion that is not transmitted to the output is diverted to a detector. The detector measures the diverted portion. A signal indicative of the amount of the diverted portion is sent to control electronics. The control electronics send a signal to the variable attenuator, thereby resetting the previous attenuation setting, to increase or decrease attenuation to maintain constant average signal power out of the optical tap.
The aforedescribed conventional variable attenuator implementation thus requires at least three separate fiber-coupled components: a variable attenuator, an optical tap (such as a fused fiber coupler having one percent tap efficiency), and a fiber-coupled detector. The conventional implementation of the variable attenuator is thus relatively complex and expensive. It would be desirable to provide the art with a variable attenuator wherein all related components are integrated into a single device thereby providing cost savings, and reducing the size and complexity of the device.